Wrenches for limiting an applied torque generally have a head portion with an extending handle. The head portion has a box or other wrench driver and may have a ratchet-like mechanism inside for rapid tightening capability. The handle is used for leverage to turn the head portion and tighten a nut or other fastener until a point at which the nut becomes tight and the handle clicks or slightly slips on application of further torque. An operator, noticing the slippage, then ceases applying further torque. Numerous mechanisms for providing this slip within a torque wrench are known. In particular, U.S. Pat. Nos. 1,814,554, 2,172,561, 2,704,472, 2,732,747, 2,743,368, 3,165,014 and 3,274,827 disclose various torque-limited wrenches.
The most prevalent type of presently available torque-limiting wrenches generally is designed for applications in which accuracy is desired but is not critical. The wrenches operate in such a manner that the user must release pressure on the handle at the exact moment the handle slips, or risk applying an extra increment of torque at the point at which the handle stops slipping. In other words, typical torque wrenches only slip a small distance before the handle again becomes rotationally coupled to the driving head by virtue of a structural handle pivot limit. It is incumbent on the user to swiftly notice the slip and stop the torque on the handle. Wrenches used in torquing car engine bolts or aircraft structural fasteners, for example, must provide a level of accuracy to ensure proper engine performance and stress wear patterns. The torque ranges for such conventional wrenches may be from 5 to 100 foot-pounds though, and accuracy within a foot-pound is considered sufficient. Although an extra amount inadvertently applied by a conventional torque wrench with a large torque capacity is relatively harmless, even a small application of excessive torque assumes a greater importance the lower the torque requirements of a particular fastener becomes.
U.S. Pat. No. 1,303,595 issue to Rifflard discloses a torque wrench having a plunger rod with a fork and a pair of V-shaped nosings shaped to fit within corresponding notches in two star wheels, the wheels being rotationally fixed to a wrench socket wheel. An internally threaded cap regulates the level of compression of a spring acting on the head of the plunger rod. Upon rotation of the handle, and when the reaction torque from a wrench socket reaches a certain point, the nosings cam out of the notches in the star wheels, halting further torquing of the socket. However, to reset the handle once it has been pivoted, the wrench socket wheel must be reoriented with respect to the handle by reducing spring pressure on the plunger rod, in a time-consuming operation.
Thus, despite the existence of prior torque-limiting wrenches, there is a need in certain specialized applications for an improved torque wrench. In dental surgery, for instance, devices for precisely tightening dental prostheses to implants are needed.
In installing a dental prosthesis, a series of drilling steps is necessary to penetrate the jaw bone and prepare the site for the dental implant. Threads are tapped into dense bone and the dental implant (analogous to the root of a tooth) is then either press-fit or screwed into the prepared hole. At this point, a titanium or gold cover screw is inserted into a threaded hole in the implant, and the adjacent tissue sutured over to bury it for three or four months and allow the surrounding bone to set around the implant. Typically, a torque of around 10 Newton-centimeters which is equal to 0.886 inch-pounds, or 0.074 foot-pounds, is used to install the cover screw.
Typically, the dental implant, after subsequent re-exposure, has osseo-integrated into the jaw bone. After removal of the titanium cover screw, an abutment securely attaches to the implant to provide a foundation for the actual dental prosthesis, or tooth. The abutment is either a one-piece screw-in type or is held on the implant with a separate screw, both installed to a particular torque limit. The torque at which the abutment is installed is around 20 Newton-centimeters which is equal to 1.77 inch-pounds, or 0.148 foot-pounds. The dental prosthesis is then usually cemented onto the abutment.
It is extremely important that the abutment be installed to proper torque limits. Clinical experiments have revealed that proper installation torque is important to the quality of the installed dental prosthesis. Too much torque may over stress or strip the bone or dental screw or abutment. Insufficient torque will not ensure a sufficient purchase into the bone possibly causing the screw or abutment to loosen, requiring a follow up re-tightening procedure. In the past, the abutments have been installed by hand with no torque-control hand tools. This process resulted in a widely varying range of torque being applied, feasibly by the same dentist on the same day. Due to the awkward position assumed by the dental surgeon, the extremely low torque limits and the non-stationary and unique work area of a patient's mouth, first estimating, and then repeating a particular application of torque is problematic at best. Typically, the surgeon is reluctant to apply too much torque and the screw or abutment subsequently works itself loose.
The present invention provides an improved torque-limiting tool for highly accurate applications which overcomes the deficiencies of prior art wrenches.